Synthesis of metal nanoparticles

ABSTRACT

Methods for synthesizing metal nanoparticles and the nanoparticles so produced are provided. The methods include addition of surfactant to a novel reagent complex between zero-valent metal and a hydride. The nanoparticles produced by the method include oxide-free, zero-valent tin nanoparticles useful in fabricating a battery electrode.

TECHNICAL FIELD

The present invention relates in general to a method of synthesizingnanoparticles of zero-valent metal, in particular nanoparticles ofzero-valent tin, and also relates in general to nanoparticles ofelemental metal, in particular nanoparticles of tin.

BACKGROUND

Metal nanoparticles, particles of elemental metal in pure or alloyedform with a dimension less than 100 nm, have unique physical, chemical,electrical, magnetic, optical, and other properties in comparison totheir corresponding bulk metals. As such they are in use or underdevelopment in fields such as chemistry, medicine, energy, and advancedelectronics, among others.

Synthetic methods for metallic nanoparticles are typically characterizedas being “top-down” or “bottom-up” and comprise a variety of chemical,physical, and even biological approaches. Top-down techniques involvethe physical breakdown of macroscale or bulk metals into nanoscaleparticles using a variety of physical forces. Bottom-up methods involvethe formation of nanoparticles from isolated atoms, molecules, orclusters.

Physical force methods for top-down metal nanoparticle synthesis haveincluded milling of macroscale metal particles, laser ablation ofmascroscale metals, and spark erosion of macroscale metals. Chemicalapproaches to bottom-up synthesis commonly involve the reduction ofmetal salt to zero-valent metal coupled with growth around nucleationseed particles or self-nucleation and growth into metal nanoparticles.

While each of these methods can be effective in certain circumstanceseach also has disadvantages or situational inapplicability. Directmilling methods can be limited in the size of particles obtainable(production of particles smaller than ˜20 nm is often difficult) and canlead to loss of control of the stoichiometric ratios of alloys. Otherphysical methods can be expensive or otherwise unamenable to industrialscale.

Chemical reduction techniques can fail in situations where the metalcation is resistant to reduction. Mn(II) for example is notoriouslyimpervious to chemical reduction. Conventional chemical reductionapproaches can also be unsuitable for producing nanoparticles forapplications that are highly sensitive to oxidation. Tin nanoparticles,for example, can be difficult to obtain from reduction approaches atsizes less than 20 nm and even when so obtained tend to contain a largeproportion of SnO₂.

Tin is a promising material for battery electrodes. For example, as ananode in a Li-ion battery, tin can store approximately three times thecharge density of the commonly used graphite anode. Recently it has beenshown that tin-based material holds great promise in use as a Mg-ioninsertion type anode for high energy density Mg-ion batteries. Inparticular, anode material fabricated from ˜100 nm tin powder achievedhigh capacity and low insertion/extraction voltage.

SUMMARY

A method for synthesizing metal nanoparticles and the nanoparticles sosynthesized are provided.

In one aspect, a method for synthesizing metal nanoparticles isdisclosed. The method includes the step of adding surfactant to areagent complex:

M⁰X_(y)  I,

wherein M⁰ is a zero-valent metal, wherein X is a hydride, and wherein yis an integral or fractional value greater than zero. As described, thereagent complex can be a reagent comprising a zero-valent metal incomplex with hydride. In some variations the reagent complex can be insuspended contact with a solvent at the time of addition. In somevariations, the hydride can be lithium borohydride, the zero-valentmetal can be tin, the surfactant can be octylamine, or any combinationthereof.

In another aspect, metal nanoparticles and their method of synthesis isdisclosed. The method includes the step of adding surfactant to areagent complex:

M⁰X_(y)  I,

wherein M⁰ is a zero-valent metal, wherein X is a hydride, and wherein yis an integral or fractional value greater than zero. As described, thereagent complex can be a reagent comprising a zero-valent metal incomplex with hydride. In some variations the reagent complex can be insuspended contact with a solvent at the time of addition. In somevariations, the hydride can be lithium borohydride, the zero-valentmetal can be tin, the surfactant can be octylamine, or any combinationthereof.

In another aspect metal nanoparticles having an average maximumdimension less than about 20 nm and which are substantially oxide-freeis disclosed. In some variations the metal nanoparticles have an averagemaximum dimension of about 10 nm, are composed substantially of Sn⁰, orboth.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects and advantages of the invention will become apparent andmore readily appreciated from the following description of theembodiments taken in conjunction with the accompanying drawings, ofwhich:

FIG. 1 is an x-ray diffraction spectrum of tin nanoparticles synthesizedby the method reported here; and

FIG. 2A is an x-ray photoelectron spectrum of Sn⁰ powder;

FIG. 2B is an x-ray photoelectron spectrum of an Sn(LiBH₄)₂ complexprepared by the process reported here; and

FIG. 2C is an overlay of the x-ray spectrum of Sn⁰ powder of FIG. 2A andthe x-ray photoelectron spectrum of an Sn(LiBH₄)₂ complex prepared bythe process of FIG. 2 of FIG. 2B.

DETAILED DESCRIPTION

A method for synthesizing metal nanoparticles, the nanoparticles sosynthesized, and compositions comprising the nanoparticles aredescribed. As explained in the following description, the methodinvolves a reaction between a surfactant and a novel reagent complexcomprising a zero-valent metal and a hydride. A “zero-valent metal” canalternatively be described as an elemental metal or as a metal which isin oxidation state zero. The novel reagent complex can alternatively bedescribed as a complex.

As used here, a “metal” can refer to an alkaline earth metal, an alkalimetal, a transition metal, or a post-transition metal. The phrase“transition metal” can refer to any D-block metal of Groups 3 through12. The phrase “post-transition metal” can refer to any metal of theGroups 13 through 16, including aluminum, gallium, indium, tin,thallium, lead, or bismuth. In some variations, a metal will be atransition metal or a post-transition metal. In some examples a metalwill be tin.

As used here, a “hydride” can be a solid metal hydride (e.g. NaH, orMgH₂), metalloid hydride (e.g. BH₃), complex metal hydride (e.g.LiAlH₄), or salt metalloid hydride also referred to as a salt hydride(e.g. LiBH₄). The term “metalloid” can refer to any of boron, silicon,germanium, arsenic, antimony, tellurium, or polonium. In some examplesthe hydride will be LiBH₄. Any member of a group consisting of complexmetal hydrides and salt metalloid hydrides can be called a “complexhydride”. It is to be appreciated that the term hydride as used hereincan also encompass a corresponding deuteride or tritide.

A method for synthesizing metal nanoparticles includes the step ofadding surfactant to a reagent complex:

M⁰−X_(y)  I,

wherein M⁰ is a zero-valent metal, wherein X is a hydride molecule, andwherein y is a value greater than zero. In many instances y can be avalue greater than zero and less than or equal to four. The valuerepresented by y can be an integral value or a fractional value, such as2.5. The complex described by Formula I is referred to alternativelyherein as a “reagent complex”

The reagent complex can be a complex of individual molecular entities,such as a single metal atom in oxidation state zero in complex with oneor more hydride molecules. Alternatively the complex described byFormula I can exist as a molecular cluster, such as a cluster of metalatoms in oxidation state zero interspersed with hydride molecules, or acluster of metal atoms in oxidation state zero, the clustersurface-coated with hydride molecules or the salt hydride interspersedthroughout the cluster.

In some aspects of the method for synthesizing metal nanoparticles, thereagent complex can be in suspended contact with a solvent or solventsystem. In some variations, suitable solvents or solvent systems willinclude those in which a suspension of the reagent complex is stable foran interval of at least one day in an inert environment. In somevariations, suitable solvents or solvent systems will include those inwhich a suspension of the reagent complex is stable for an interval ofat least one hour in an inert environment. In some variations, suitablesolvents or solvent systems will include those in which a suspension ofthe reagent complex is stable for an interval of at least five minutesin an inert environment.

The phrase “an inert environment” as used here can include anatmospheric environment that is anhydrous. The phrase “an inertenvironment” as used here can include an atmospheric environment that isoxygen-free. The phrase “an inert environment” as used here can includean atmospheric environment that is both anhydrous and oxygen-free. Thephrase “an inert environment” as used here can include enclosure in anambient atmosphere comprising an inert gas such as argon, or enclosurein a space that is under vacuum.

The term “stable” as used in the phrase, “in which the reagent complexis stable for an interval” can mean that the reagent complex does notappreciably dissociate or undergo covalent transformation.

The solvent or solvent system employed in certain various aspectsdisclosed here can be a material that is non-reactive toward the hydrideincorporated into the reagent complex. As used above in the phrase“material that is non-reactive toward the hydride”, the term“non-reactive” can mean that the material, i.e. the solvent or solventsystem, does not directly participate in or bring about covalentreaction of the hydride of the reagent complex to a thermodynamicallysignificant extent. According to such a criterion, suitable solvents orsolvent systems can vary depending on the hydride being used. In somevariations this can include a solvent or solvent system that is aprotic,non-oxidative or both.

Non-limiting examples of suitable solvents or solvent system componentscan include acetone, acetonitrile, benzene, 1-butanol, 2-butanol,2-butanone, t-butyl alcohol, carbon tetrachloride, chlorobenzene,chloroform, cyclohexane, 1,2-dichloroethane, diethyl ether, diethyleneglycol, diglyme (diethylene glycol, dimethyl ether),1,2-dimethoxy-ethane (glyme, DME), dimethylether, dimethyl-formamide(DMF), dimethyl sulfoxide (DMSO), dioxane, ethanol, ethyl acetate,ethylene glycol, glycerin, heptane, Hexamethylphosphoramide (HMPA),Hexamethylphosphorous triamide (HMPT), hexane, methanol, methyl t-butylether (MTBE), methylene chloride, N-methyl-2-pyrrolidinone (NMP),nitromethane, pentane, Petroleum ether (ligroine), 1-propanol,2-propanol, pyridine, tetrahydrofuran (THF), toluene, triethyl amine,o-xylene, m-xylene, or p-xylene.

As non-limiting examples, in some instances a halogenated alkyl solventcan be acceptable, in some instances an alkyl sulfoxide can beacceptable, in other instances an ethereal solvent can be acceptable. Insome variations THF can be a suitable solvent or solvent systemcomponent.

In some aspects of the method for synthesizing metal nanoparticles, thesurfactant can be suspended or dissolved in a solvent or solvent system.In different variations wherein the reagent complex is in suspendedcontact with a solvent or solvent system and the surfactant is suspendedor dissolved in a solvent or solvent system, the reagent complex can bein suspended contact with a solvent or solvent system of the same ordifferent composition as compared to the solvent or solvent system inwhich the surfactant is dissolved or suspended.

In some aspects of the method for synthesizing metal nanoparticles, thereagent complex can be combined with surfactant in the absence ofsolvent. In some such cases a solvent or solvent system can be addedsubsequent to such combination. In other aspects, surfactant which isnot suspended or dissolved in a solvent or solvent system can be addedto a reagent complex which is in suspended contact with a solvent orsolvent system. In yet other aspects, surfactant which is suspended ordissolved in a solvent or solvent system can be added to a reagentcomplex which is not in suspended contact with a solvent or solventsystem.

The surfactant can be any known in the art. Usable surfactants caninclude nonionic, cationic, anionic, amphoteric, zwitterionic, andpolymeric surfactants and combinations thereof. Such surfactantstypically have a lipophilic moiety that is hydrocarbon based,organosilane based, or fluorocarbon based. Without implying limitation,examples of types of surfactants which can be suitable include alkylsulfates and sulfonates, petroleum and lignin sulfonates, phosphateesters, sulfosuccinate esters, carboxylates, alcohols, ethoxylatedalcohols and alkylphenols, fatty acid esters, ethoxylated acids,alkanolamides, ethoxylated amines, amine oxides, alkyl amines, nitriles,quaternary ammonium salts, carboxybetaines, sulfobetaines, or polymericsurfactants.

In some instances the surfactant employed in the method for synthesizingmetal nanoparticles will be one capable of oxidizing, protonating, orotherwise covalently modifying the hydride incorporated in the reagentcomplex. In some variations the surfactant can be a carboxylate,nitrile, or amine. In some examples the surfactant can be octylamine.

In some variations the method for synthesizing metal nanoparticles canbe performed under an anhydrous environment, under an oxygen-freeenvironment, or under an environment that is anhydrous and oxygen-free.For example, the method for synthesizing metal nanoparticles can beperformed under argon gas or under vacuum. While the zero-valent metalM⁰ can contain some impurities such as metal oxides, the method forsynthesizing metal nanoparticles can in some instances produce puremetal nanoparticles, free of oxide species. Such an instance is shown inFIG. 1, an x-ray diffraction spectrum of zero-valent tin nanoparticlesproduced by the method. It is to be noted that the diffraction spectrumof FIG. 1 indexes to pure, zero-valent tin, free of oxides and measuresan average maximum particle dimension of 11 nm.

The complex described by Formula I can be produced by any suitableprocess. A non-limiting example of a suitable process for preparing thereagent complex includes a step of ball-milling a hydride with apreparation composed of a zero-valent metal. A process employing thisstep for production of a reagent complex will be referred to herein asthe “example process”. In many instances the preparation composed of azero-valent metal employed in the example process will have a highsurface-area-to-mass ratio. In some instances the preparation composedof a zero-valent metal will be a metal powder. It is contemplated thatthe preparation composed of a zero-valent metal could be a highly porousmetal, a metal with a honeycomb structure, or some other preparationwith a high surface-area-to-mass ratio.

In some instances the preparation containing a zero-valent metal caninclude a zero-valent transition metal. Suitable transition metalsinclude, but are not limited to cadmium, cobalt, copper, chromium, iron,manganese, gold, silver, platinum, titanium, nickel, niobium,molybdenum, rhodium, palladium, scandium, vanadium, and zinc. In someinstances the preparation containing a zero-valent metal can include azero-valent post-transition metal. Suitable post-transition metalsinclude aluminum, gallium, indium, tin, thallium, lead, or bismuth.

It is to be understood that the zero-valent metal, be it transitionmetal, post-transition metal, alkali metal, or alkaline earth metal,will be in oxidation state zero. As used herein, “zero-valent” and“oxidation state zero” are taken to mean that the material can exhibit asubstantial, but not necessarily complete, zero oxidation state. Forexample, the preparation containing a zero-valent metal can include somesurface impurities such as oxides.

It is contemplated that the phrase “high-surface-area-to-mass ratio” canencompass a broad range of surface-area-to-mass ratios and, in general,the surface-area-to-mass ratio of the preparation composed of azero-valent metal employed will be that which is required by the timeconstraints of the example process. In many instances, a highersurface-area-to-mass ratio of the preparation composed of a zero-valentmetal will lead to more rapid completion of the example process. In thecase where the preparation composed of a zero-valent metal is a metalpowder for example, smaller particle size of the metal powder can tendto lead to more rapid completion of the example process and resultantproduction of the reagent complex.

Non-limiting examples of hydrides suitable for use in the exampleprocess include sodium borohydride, lithium aluminium hydride,diisobutylaluminium hydride (DIBAL), Lithium triethylborohydride (superhydride), sodium hydride and potassium hydride, calcium hydride, lithiumhydride, or borane.

In some variations of the example process, the hydride can be mixed withthe preparation composed of a zero-valent metal in a 1:1 stoichiometricratio of hydride molecules to metal atoms. In other variations thestoichiometric ratio can be 2:1, 3:1, 4:1 or higher. In some variationsthe stoichiometric ratio of hydride molecules to metal atoms in thepreparation composed of a zero-valent metal can also include fractionalquantities, such as 2.5:1. It is to be understood that, in cases wherethe example process is employed for production of the reagent complex,the stoichiometry of admixture in the example process will tend tocontrol the stoichiometry of the complex according to Formula I asindicated by the value of y.

It is contemplated that a ball mill used in the example process can beof any type. For example the ball mill employed can be a drum ball mill,a jet mill, a bead mill, a horizontal rotary ball mill, a vibration ballmill or a planetary ball mill. In some examples the ball mill employedin the example process will be a planetary ball mill.

It is contemplated that the ball-milling media used in the exampleprocess can be of any composition. For example, the ball-milling mediaemployed can be composed of metal such as stainless steel, brass, orhardened lead or they can be composed of ceramic such as alumina orsilica. In some variations the ball milling media in the example processwill be stainless steel. It is to be appreciated that the ball-millingmedia can be of a variety of shapes, for example they can be cylindricalor spherical. In some variations the ball-milling media will bespherical.

Optionally, a variety of analytical techniques can be employed tomonitor the example process and to determine successful completionthereof. Some such techniques, such as x-ray photoelectron spectroscopy(XPS) and x-ray diffraction (XRD) are discussed below, but anyanalytical approach known to be useful in the art can be optionallyemployed.

XPS scans in the tin region are shown for elemental tin powder and for areagent complex Sn(LiBH₄)₂, in FIGS. 2A and 2B, respectively. In FIGS.2A and 2B, heavy solid lines show the raw XPS data and light solid linesshow the adjusted data. Dashed and/or dotted lines show the individualdeconvoluted peaks of the spectra. The center points in electronVolts ofdeconvoluted peak maxima are indicated by arrows.

FIG. 2C shows an overlay of the adjusted spectrum of uncomplexed tin(dotted line), from FIG. 2A, with the adjusted spectrum of theSn(LiBH₄)₂ complex (solid line), from FIG. 2B. As can be seen in FIG.2C, complex formation between the zero-valent tin and the lithiumborohydride results in the appearance of new peaks and a general shiftof the spectrum toward lower electronic energy of the observed electronsof the zero-valent metal. In some instances where the reagent complex isprepared by the example process, x-ray photoelectron spectra of thezero-valent metal incorporated in the reagent complex will be generallyshifted toward lower energy as compared to the spectra of theuncomplexed zero-valent metal. In some instances, reagent complexeswherein M⁰ is tin and X is lithium borohydride can be identified by thepresence of an x-ray photoelectron spectroscopy peak centered at about484 eV.

In some variations, the example process can be performed under ananhydrous environment, an oxygen-free environment, or an anhydrous andoxygen-free environment. For example, the example process can beperformed under argon gas or under vacuum. This optional feature can beincluded, for example, when the hydride used in the example process is ahydride that is sensitive to molecular oxygen, water, or both.

As mentioned, Mg-ion batteries employing tin-based anodes have shownpromise as high energy density alternatives to conventional Li-ionbatteries (N. Singh et al., Chem. Commun., 2013, 49, 149-151;incorporated by reference herein in its entirety). In particular, tinanodes based on ˜100 nm Sn⁰ powder have shown impressive capacity andinsertion/extraction voltage in such systems. A dramatic decrease in thetin nanostructure of such an anode can improve such a system's ratecapability and cyclability, but requires tin nanoparticles which areoxide-free. Tin nanoparticles such as 11 nm oxide free tin nanoparticlesdisclosed here and represented in FIG. 1 can be a useful anode materialin such a battery system.

The present invention is further illustrated with respect to thefollowing examples. It needs to be understood that these examples areprovided to illustrate specific embodiments of the present invention andshould not be construed as limiting the scope of the present invention.

EXAMPLE 1

0.503 g of tin metal powder and 0.187 g of lithium borohydride arecombined in a planetary ball mill. The combination is ball-milled in aplanetary ball-mill for 4 hours at 400 rpm (using a Fritsch pulervisette7 planetary ball mill) in a 250 mL stainless steel airtight ball-milljar with 1¾ inch, 3½ inch, and 5¼ inch 316 stainless steel ballbearings. The resulting ball-milled complex is suspended in THF. Thesuspension is titrated with a solution of 0.443 g octylamine in 10 mL ofTHF. The ensuing reaction proceeds at ambient temperature to completionin approximately 3 hours, resulting in zero-valent tin nanoparticleswith an average grain size of about 11 nm, as shown in the x-raydiffraction spectrum of FIG. 1. The spectrum of FIG. 1 indexes to puretin metal that is free of oxide species.

The foregoing description relates to what are presently considered to bethe most practical embodiments. It is to be understood, however, thatthe disclosure is not to be limited to these embodiments but, on thecontrary, is intended to cover various modifications and equivalentarrangements included within the spirit and scope of the appendedclaims, which scope is to be accorded the broadest interpretation so asto encompass all such modifications and equivalent structures as ispermitted under the law.

What is claimed is:
 1. A method for synthesizing metal nanoparticles,comprising: adding surfactant to a reagent complex according to FormulaI,M⁰X_(y)  I, wherein M⁰ is a zero-valent metal, X is a hydride, and y isan integral or fractional value greater than zero.
 2. The method ofclaim 1 wherein the reagent complex is in suspended contact with asolvent.
 3. The method of claim 2 wherein the solvent is an etherealsolvent.
 4. The method of claim 3 wherein the ethereal solvent istetrahydrofuran.
 5. The method of claim 1 wherein the zero-valent metalis a zero-valent post-transition metal.
 6. The method of claim 5 whereinthe period 5 zero-valent post-transition metal is tin.
 7. The method ofclaim 6 wherein the complex has an x-ray photoelectron spectroscopy peakcentered at about 484 eV.
 8. The method of claim 1 wherein thesurfactant comprises an aliphatic amine.
 9. The method of claim 8wherein the surfactant comprises octylamine.
 10. The method of claim 1wherein the hydride is a salt metalloid hydride.
 11. The method of claim10 wherein the hydride is a borohydride.
 12. The method of claim 11wherein the hydride is lithium borohydride.
 13. The method of claim 1further comprising the step, prior to adding surfactant to the reagentcomplex, of: ball-milling a mixture of a hydride with a preparationcomposed of a zero-valent metal.
 14. The method of claim 13 wherein thepreparation composed of a zero-valent metal is a preparation of tin. 15.The method of claim 13 wherein the hydride is a salt metalloid hydride.16. The method of claim 15 wherein the hydride is a borohydride.
 17. Themethod of claim 16 wherein the hydride is lithium borohydride.
 18. Themethod of claim 13 wherein the hydride and the preparation composed of azero-valent metal are mixed in a stoichiometric ratio of hydridemolecules to zero-valent metal atoms of about 1:1, 2:1, 3:1, 4:1, orintermediate ratios.
 19. Metal nanoparticles synthesized by a methodcomprising: adding surfactant to a reagent complex according to FormulaI,M⁰X_(y)  I, wherein M⁰ is a zero-valent metal, X is a hydride, and y isan integral or fractional value greater than zero.
 20. The metalnanoparticles of claim 19 wherein the reagent complex is in suspendedcontact with a solvent.
 21. The metal nanoparticles of claim 19 whereinthe surfactant comprises octylamine.
 22. The metal nanoparticles ofclaim 19 consisting essentially of a post-transition metal.
 23. Themetal nanoparticles of claim 22 wherein the post-transition metal is tinand the nanoparticles have an average maximum dimension less than about20 nm.
 24. The zero-valent nanoparticles of claim 23 which aresubstantially oxide free and characterized in that their average maximumdimension is about 10 nm.
 25. Metal nanoparticles having an averagemaximum dimension of less than about 20 nm and consisting essentially ofa zero-valent metal which is substantially oxide free.
 26. The metalnanoparticles of claim 25 wherein the average maximum dimension is lessthan or equal to about 10 nm.
 27. The metal nanoparticles of claim 25wherein the metal nanoparticles consist essentially of tin.
 28. Abattery electrode comprising the metal nanoparticles of claim 26.